Ligation of Fas and Cell Death
The TNF plot thickens
3 July 2000
The signal transduction pathways activated by TNF-alpha are more complicated than those activated by Fas ligand (FasL) for a number of reasons. First, there are of course two receptors for TNF rather than one. Addition of TNF usually activates NF-kappa B (requiring the adapter protein RIP) and Jun [requiring other adapter proteins, the TRAFs (tumor necrosis factor receptor-associated factors)], whereas activation of these pathways is not always seen after Fas ligation. Fas can directly associate with the Fas-associated death domain protein (FADD), leading to activation of caspase 8, whereas TNF receptor type 1 (TNF-R1) seems to associate with FADD only indirectly, via TRADD (TNF-R1-associated death domain protein). Activated NF-kappa B has been shown to transactivate a number of anti- apoptotic genes, including Bcl-xL, the Bcl-2 family member A1, and the inhibitor-of-apoptosis proteins IAP1 and IAP2.
It has been proposed that because TNF induces expression of cell death inhibitors (via NF-kappa B) as well as caspase activation (via FADD), many cells don't die when their TNFRs are ligated (1,2). This model does explain why cycloheximide together with TNF is a much more potent inducer of cell death, and why cells lacking components of NF-kappa B are more likely to die when exposed to TNF, but there are still some problems. One is the difficulty of explaining how a pathway requiring transcription and translation manages to beat a short pathway involving direct association of TRADD, FADD, and caspase 8.
Getting more to the point of the questions raised by Dr Jelinek, in most cases addition of TNF does not cause activation of any caspases, unless cycloheximide is also present. When apoptosis is induced by TNF it can usually be inhibited by CrmA (cowpox virus cytokine response modifier A), an inhibitor of caspases 1 and 8, but rarely by Bcl-2 or Bcl-xL.
Point 2 is an excellent one. (I wish I knew the answer!) I don't see how NF-kappa B-transactivated proteins could get to the mitochondria in time, so I would have to guess that maybe other proteins or pathways are involved. For example, maybe the low, steady-state levels of NF-kappa B are required for production of a short lived protein that can act in a rapid post- transcriptional pathway to inhibit caspase activation when TNF receptors are ligated (3). What might this protein be?
On a different (but related) note, a recent paper may help answer the question of why TNF alone rarely causes apoptosis in vitro, and why it causes necrosis rather than apoptosis when used in vivo. This paper (4) shows that in vivo, TNF causes death of tumor cells indirectly, by acting on the blood vessels that feed them. The tumor cells die a necrotic death due to loss of blood supply, rather than apoptosing cell-autonomously.
1 Beg, A.A. and Baltimore, D. (1996). An essential role for NF-kappa -B in preventing TNF-alpha-induced cell death. Science 274, 782-784.[Medline]
2 Wang, C.Y., Mayo, M.W. and Baldwin, A.S. (1996). TNF- and cancer therapy-induced apoptosis - potentiation by inhibition of NF-kappa-B. Science 274, 784-787.[Medline]
3 Kajino, S., Suganuma, M., Teranishi, F., Takahashi, N., Tetsuka, T., Ohara, H., Itoh, M. and Okamoto, T. (2000). Evidence that de novo protein synthesis is dispensable for anti-apoptotic effects of NF-kappa B. Oncogene 19, 2233-2239.[Medline]
4 Stoelcker, B., Ruhland, B., Hehlgans, T., Bluethmann, H., Luther, T. and Mannel, D.N. (2000). Tumor necrosis factor induces tumor necrosis via tumor necrosis factor receptor type 1-expressing endothelial cells of the tumor vasculature. Am. J. Pathol. 156, 1171-1176.[Medline]
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